Perpendicular magnetic recording disk

1. A perpendicular magnetic recording disk including an underlayer of Ti between a substrate and a perpendicular magnetic recording layer of CoCr alloy for inducing perpendicular orientation of the perpendicular magnetic recording layer, and an intermediate soft magnetic layer of NiFe between the underlayer and the perpendicular magnetic recording layer for forming closed magnetic loops together with the perpendicular magnetic recording layer, the perpendicular magnetic recording layer having a thickness in the range of 20-5 nm where the ratio of perpendicular coercivity Hc to maximum perpendicular coercivity Ho decreases with reduced thickness of the perpendicular magnetic recording layer.

2. The perpendicular magnetic recording disk of claim 1 , wherein, in the range of thickness of the perpendicular magnetic recording layer, the rate of variation of the ratio of perpendicular remanent magnetization of maximum perpendicular remanent magnetization is greater than of the ratio of perpendicular coercivity Hc to maximum perpendicular coercivity Ho.

3. The perpendicular magnetic recording disk of claim 1 , wherein, in the range of thickness of the perpendicular magnetic recording layer, a noise level constant of proportionality a expressed as the following formula decreases with reduced thickness of the perpendicular magnetic recording layer: α = 4 ⁢ π ⁢ ⁢ Mr Hc where Mr is the perpendicular remanent magnetization and Hc is the perpendicular coercivity.

4. The perpendicular magnetic recording disk of claim 1 , wherein the perpendicular magnetic recording layer further comprises at least one material selected from the group consisting of B, Pt, Ta, V, Nb, Zr, Y, and Mo.

5. A perpendicular magnetic recording disk including an underlayer of Ti between a substrate and a perpendicular magnetic recording layer for inducing perpendicular orientation of the perpendicular magnetic recording layer, and an intermediate soft magnetic layer of NiFe between the underlayer and the perpendicular magnetic recording layer for forming closed magnetic loops together with the perpendicular magnetic recording layer, the perpendicular magnetic recording layer having a thickness in the range where the ratio of perpendicular coercivity Hc to maximum perpendicular coercivity Ho decreases with reduced thickness of the perpendicular magnetic recording layer; wherein the perpendicular magnetic recording layer is formed of a CoCr alloy including at least one material selected from the group consisting of B, Pt, Ta, V, Nb, Zr, Y, and Mo and has a thickness of 20-50 nm.

6. The perpendicular magnetic recording disk of claim 1 , wherein the intermediate soft magnetic layer further comprises at least one material selected from the group consisting of Nb, V, Ta, Zr, Hf, Ti, B, Si, and P.

7. The perpendicular magnetic recording disk of claim 6 , wherein the intermediate soft magnetic layer has a thickness of 3-30 nm.

8. The perpendicular magnetic recording disk of claim 1 , further comprising a protective layer and a lubricant layer sequentially on the perpendicular magnetic recording layer.